The overall goal of these studies is to determine the molecular mechanism(s) which regulate antithrombin (AT-III) levels during development. The major aim of the proposed studies is to determine if changes in chicken AT-III levels during fetal and neonatal development reflect regulation of AT-III gene expression. Changes in circulating AT-III levels that occur during chick embryo development are reflected in both antigen levels and the degree of sialation of the molecule. The increase in AT- III antigen levels and the degree of sialation of the molecule. The increase in AT-III antigen levels during chick embryo development is completely analogous to changes in AT-III molecule synthesized by chickens is structurally and functionally very similar to human AT-III. Furthermore, the chick embryo offers a model system for examining changes in gene expression in vivo and in vitro in a highly characterized chick hepatocyte culture system. To determine whether the change in AT-III protein levels is due to changes in AT-III gene expression, we will: 1) Develop a cDNA clone(s) to chicken AT-III. 2) Use this clone to quantify changes in AT-III mRNA with AT-III antigen levels by measuring steady- state AT-III mRNA levels by slot blot and Sl nuclease hybridization analysis. A third aim is to assess whether a synthetic glucocorticoid dexamethasone can stimulate circulating AT-III levels at different developmental stages. Our goal is to further our understanding of the physiological events which modulate circulating AT-III levels. These studies are the first step in elucidating the molecular mechanism(s) responsible for the regulation of AT-III gene expression during the development of newborns. An understanding of the mechanism(s) involved in this process will allow for future studies in which the use of agents such as glucocorticoid hormones can be evaluated for their potential therapeutic use in humans. The aim is to be able to induce of stimulate the level of AT-III in vivo to specifically alter normal developmental patterns of AT-III expression. This will be particularly useful in premature and newborn infants suffering from hypercoagulation due to AT-III insufficiency.